v1.0.12: Fix link MTU clamping, echo-back prevention, oversized frame detection

MTU Clamping (Bug 8a): Clamp link MTU signalling in LINKREQUEST packets
when forwarding through transport node, matching Python reference impl.
Without this, TCP endpoints negotiate 8192-byte segments that exceed the
V3's 1064-byte HDLC buffer, causing silent truncation and permanent
resource transfer stalls at ~70%.

Fixed MTU declaration (Bug 8b): Set FIXED_MTU=true on TcpInterface so
Transport uses HW_MTU for clamping decisions.

Oversized frame detection (Bug 8c): Track truncated HDLC frames and
drop them with a diagnostic log instead of silently delivering corrupt
data to Transport.

Echo-back prevention (v1.0.10): Track which TCP client originated each
inbound frame and skip that client in send_outgoing() to prevent
flooding TCP buffers.

Register local client interface: Enable Transport forwarding of
announces, link packets, and proofs to TCP clients.

Document all discovered microReticulum bugs in MICRORETICULUM_BUGS.md.

RNode_Firmware.ino

@@ -711,6 +711,9 @@ void setup() {
         local_tcp_rns_interface = local_tcp_interface_ptr;
         local_tcp_rns_interface.mode(RNS::Type::Interface::MODE_GATEWAY);
         RNS::Transport::register_interface(local_tcp_rns_interface);
+        // Register as local client interface so Transport forwards
+        // announces, link packets, and proofs to TCP clients
+        RNS::Transport::register_local_client_interface(local_tcp_rns_interface);
 
         {
           char _bm_msg[128];